1. Field of the Invention
Embodiments of the present invention generally relate to photovoltaic/solar cell and solar panel manufacturing.
2. Description of the Related Art
Photovoltaics (PV) systems can generate power for many uses, such as remote terrestrial applications, battery charging for navigational aids, telecommunication equipments, and consumer electronic devices, such as calculators, watches, radios, etc. One example of PV systems includes a stand-alone system which generates power for direct use or with local storage. Another type of PV system is connected to conventional utility grid with the appropriate power conversion equipment to produce alternating current (AC) compatible with any conventional utility grid.
PV or solar cells are material junction devices which convert sunlight into direct current (DC) electrical power. When exposed to sunlight (consisting of energy from photons), the electric field of solar cell p-n junctions separates pairs of free electrons and holes, thus generating a photo-voltage. A circuit from n-side to p-side allows the flow of electrons when the solar cell is connected to an electrical load, while the area and other parameters of the PV cell junction device determine the available current. Electrical power is the product of the voltage times the current generated as the electrons and holes recombine.
Currently, solar cells and PV panels are manufactured by starting with many small silicon sheets or wafers as material units and processing them into individual photovoltaic cells before they are assembled into PV modules and solar panels. These silicon sheets are generally saw-cut p-type boron doped silicon sheets less than about 0.3 mm thick, precut to the sizes and dimensions that will be used, e.g., 100 mm×100 mm, or 156 mm×156 mm. The cutting (sawing) or ribbon formation operation on the silicon sheets damages the surfaces of the precut silicon sheets to some degree, and etching processes using, for example, alkaline or acid etching solutions are performed on both surfaces of the silicon sheets to remove about ten to twenty microns of material from each surface and provide textures thereon.
Junctions are then formed by diffusing an n-type dopant onto the precut p-type silicon sheets, generally performed by phosphorus diffusion as phosphorus is widely used as the n-type dopant for silicon in solar cells. One phosphorus diffusion process includes coating phosphosilicate glass compounds onto the surface of the silicon sheets and performing diffusion/annealing inside a furnace. Another example of diffusing a phosphorus dopant into silicon includes bubbling nitrogen gas through liquid phosphorus oxychloride (POCl3) sources which are injected into an enclosed quartz furnace loaded with batch-type quartz boats containing the silicon sheets. Typically, a high temperature between about 850° C. and about 1,050° C. is needed to form and create a p-n junction depth of about 0.1 microns up to about 0.5 microns.
Following dopant diffusion, a phosphorus-doped SiO2 layer formed during the diffusion is generally removed with a wet etch. One or both surfaces of a PV cell can also be coated with suitable dielectrics after the p-n junction is formed. Dielectric layers are used to minimize surface charge carrier recombination and some dielectric materials, such as silicon dioxide, titanium dioxide, or silicon nitride, can be provided as antireflective coating to reduce reflection losses of photons.
The front or sun facing side of the PV cell is then covered with an area-minimized metallic contact grid for transporting current and minimizing current losses due to resistance through silicon-containing layers. Some blockage of sunlight or photons by the contact grid is unavoidable but can be minimized. The bottom of the PV cell is generally covered with a back metal which provides contact for good conduction as well as high reflectivity. Metal grids with patterns of conductive metal lines are used to collect current. Generally, screening printing thick-film technology is used in the PV cell industry to layer a conductive paste of metal materials, e.g., silver, etc., into a desired pattern and deposit a metal material layer to the surface of the silicon sheets or substrates for forming metal contact fingers or wiring channels on the front and/or back side of the solar cell. Other thin film technologies may be used for contact formation or electrode processing. The deposited metal layer, formed into contacts, is often dried and then fired or sintered at high temperature to form into good conductors in direct contact with underlying silicon materials, and a single PV cell is made. Generally, both silver and aluminum are contained in the screen printing paste for forming back side contacts with good contact conductor to silicon material and easy soldering.
Manufacturing high efficiency solar cells at low cost (providing low unit cost per Watt) is the key to making solar cells more competitive in the generation of electricity for mass consumption. Even small improvements in cost per Watt substantially increases the size of the available market. The efficiency of solar cells is directly related to the ability of a cell to collect charges generated from absorbed photons in the various layers. When electrons and holes re-combine, the incident solar energy is re-emitted as heat or light. The need to improve the efficiency of solar cells through use of low cost processes remains.